The present application relates to a polarizing element having durability against intense light and a liquid crystal projector using the polarizing element.
In a liquid crystal display device, it is necessary to dispose at least one polarizing plate at a liquid crystal panel surface based on an image forming principle. The function of the polarizing plate is to absorb one of two polarized components (so-called P polarized wave and S polarized wave) perpendicular to each other and to transmit the other component. As the polarizing plate described above, a dichroic polarizing plate in the form of a film containing an iodine-based or a dye-based high molecular weight organic material has been frequently used in the past.
As a general method for manufacturing a dichroic polarizing plate, a method has been used having the steps of dyeing a polyvinyl alcohol-based film with a dichroic material, such as iodine, and then performing crosslinking using a crosslinking agent, followed by performing uniaxial drawing. Since being formed by the drawing as described above, this type of polarizing plate is liable to shrink. In addition, since a polyvinyl alcohol-based film is formed of a hydrophilic polymer, particularly under humidified conditions, the film is very liable to deform. In addition, since the film is used, the mechanical strength thereof is inevitably insufficient to be used as an element. In order to avoid the above problem, a method for using a transparent protective film may be used in some cases.
In recent years, liquid crystal display devices have been increasingly used in various applications, and the performances of the devices have also been improved. Concomitant with the trend described above, individual elements forming the liquid crystal display devices are requested to have high reliability and durability. For example, in a liquid crystal display device, such as a transmission type liquid crystal projector, using a light source having a large quantity of light, a polarizing plate receives intense radiation. Hence, the polarizing plate used in the above device as described above is requested to have superior heat resistance. However, since the film-based polarizing plate described above is formed of an organic material, improvement in properties thereof has been naturally limited to a certain level.
In order to solve the problem described above, an inorganic polarizing plate having superior heat resistance has been sold under the trade name “Polarcor” by Corning Inc., USA. This polarizing plate is formed of silver particles dispersed in glass and does not use an organic material such as a film, and the principle of this polarizing plate is to use plasma resonance of island-shaped particles. That is, light absorption caused by surface plasma resonance which occurs when light is incident on island-shaped particles of a noble metal or a transition metal is used, and an absorption wavelength is influenced by the particle shape and the dielectric constant of the surrounding material. When the island-shaped particle has an ellipsoid shape, since resonance wavelengths in the long-axis and the short-axis directions are different from each other, polarization properties are obtained thereby; in particular, a polarized component parallel with the long axis at a long wavelength side is absorbed, and a polarized component parallel with the short axis is transmitted. However, in the case of Polarcor, a wavelength region in which the polarization properties are obtained is a region in the vicinity of an infrared region, and a visible light region requested for liquid crystal display devices is not included. This is because of the physical properties of silver used for the island-shaped particles.
In U.S. Pat. No. 6,772,608, a UV polarizing plate formed by precipitating particles in glass by thermal reduction using the above principle has been disclosed, and as a particular example, silver used as metal particles has also been disclosed. In this case, it is believed that absorption in the short axis direction is used, which is different from the case of Polarcor described above. Although the polarizing plate functions at around 400 nm as shown in FIG. 1, since the extinction ratio is small and an absorption band is very narrow, a polarizing plate capable of covering the entire visible light region may not be obtained even if Polarcor and the technique of U.S. Pat. No. 6,772,608 are used in combination.
In addition, in J. Opt. Soc. Am. A Vol. 8, No. 4, pp. 619 to 624, a theoretical analysis of an inorganic polarizing plate using plasma resonance of metal island-shaped particles has been disclosed. According to this document, it has been described that a resonance wavelength of aluminum particles is shorter than that of silver particles by approximately 200 nm, and hence when aluminum particles are used, a polarizing plate, which can be used in a visible light region, is probably manufactured.
In addition, in Japanese Unexamined Patent Application Publication No. 2000-147253, various methods for forming a polarizing plate using aluminum particles have been disclosed. Among the above methods, it has been disclosed that glass primarily formed of silicate is not preferable as a substrate since reaction occurs between the glass and aluminum, and calcium aluminoborate glass is suitably used (in paragraphs 0018 and 0019). However, glass formed of silicate has been widely commercially used as an optical glass, and highly reliable products thereof are available at a reasonable price; hence, when the glass formed of silicate is not suitably used, it is disadvantageous from an economical point of view. In addition, a method for forming island-shaped particles by etching using a resist pattern has also been disclosed (paragraphs 0037 and 0038). A polarizing plate used in a projector is generally requested to have a size of approximately several centimeters and a high extinction ratio. Accordingly, in order to form a visible-light polarizing plate, a resist pattern size is requested to be sufficiently smaller than a visible light wavelength, that is, to be several tens of nanometers, and in addition, in order to obtain a high extinction ratio, a pattern is preferably formed to have a high density. In addition, in order to use a polarizing plate for a projector purpose, a polarizing plate having a large area is desirably formed. However, as a method for forming a high density fine pattern by lithography, disclosed in this patent document, electron beam lithography is to be desirably used in order to obtain the pattern as described above. However, since the electron beam lithography is a method for drawing each pattern using electron beams, the productivity is inferior, and hence this technique is not practical.
In addition, in Japanese Unexamined Patent Application Publication No. 2001-147253, it has been disclosed that aluminum is removed by chlorine plasma; however, in general, when etching is performed as described above, chlorides adhere to sidewalls of an aluminum pattern. The chlorides may be removed by a commercially available wet etching liquid (such as SST-A2 by Tokyo Ohka Kogyo Co., Ltd.); however, since this type of chemical liquid, which reacts with aluminum chloride compounds, also reacts with aluminum although the etching rate is slow, it is difficult to realize a desired pattern shape by the method described above.
Furthermore, in Japanese Unexamined Patent Application Publication No. 2000-147253, as another method, a method has been disclosed in which aluminum is deposited on a patterned photoresist by oblique deposition, followed by removing the photoresist (paragraphs 0045 and 0047). However, it is believed that in order to ensure adhesion between a substrate and aluminum, aluminum is also preferably deposited on the substrate to a certain extent. However, it means that the shape of the aluminum film thus deposited is different from a prolate spheroid, such as a prolate ellipsoid, which is a suitable shape disclosed in paragraph 0015. In addition, in paragraph 0047, it has been disclosed that by anisotropic etching performed perpendicular to the surface, an excess deposit is removed. In order to obtain the function as the polarizing plate, shape anisotropic properties of aluminum are significantly important. Hence, it is believed important to adjust the amount of aluminum deposited on the resist portion and that on the substrate surface by etching to obtain a desired shape; however, it may be very difficult to control the amount of aluminum having a size of submicron or less, such as 0.05 μm, as disclosed in paragraph 0047, and hence it is questionable whether the method described above is a highly productive manufacturing method. In addition, as properties of the polarizing plate, a high transmittance is desirable in the transmission axis direction; however, when glass is used as the substrate, in general, several percentage of light is inevitably reflected on the glass interface, and since countermeasures have not been taken therefor, a high transmittance is difficult to obtain.
In addition, according to Japanese Unexamined Patent Application Publication No. 2002-372620, a polarizing plate formed by oblique deposition has been disclosed. This method is to obtain polarization properties by forming fine columnar structures by oblique deposition using a transparent and an opaque substance with respect to wavelengths in a service bandwidth, and since a fine pattern can be easily obtained by this method unlike the method disclosed in U.S. Pat. No. 6,772,608, it is believed that the method has a high productivity; however, problems still exist. That is, the aspect ratio of a fine columnar structure which is first formed from the substance opaque to the wavelengths in the service bandwidth, the distance between the fine columnar structures, and the linearity thereof are important factors to obtain superior polarization properties and are to be intentionally controlled in view of reproducibility of the properties. However, in this method, since the columnar structures are formed by a phenomenon in which initial deposited layers made of deposition particles form shadow areas, and following flying particles are not deposited on the shadow areas, it has been difficult to intentionally control the factors described above. As a method for improving the above situation, a method for forming polishing marks in the substrate by rubbing performed before deposition has been described; however, the particle diameter of the deposition film is approximately at most several tens of nanometers, and in order to control the anisotropic properties of this type of particles, it might be desired to intentionally form pitches on the order of submicron or less. However, by general polishing sheets or the like, pitches on the order of approximately submicron are the limit, and hence fine polishing marks as described above are difficult to form by rubbing. In addition, since the resonance wavelength of Al particles largely depends on the refractive index of the surrounding material, as described above, in this case, combination between the transparent and the opaque substances is important; however, in Japanese Unexamined Patent Application Publication No. 2002-372620, the combination to obtain superior polarization properties in a visible light region has not been described. In addition, as is the case disclosed in U.S. Pat. No. 6,772,608, when glass is generally used as the substrate, several percentage of light is inevitably reflected on the glass interface, and countermeasures have not been taken therefor.
In addition, in Applied Optics Vol. 25, No. 2, 1986, pp. 311 to 314, a polarizing plate for infrared communication, which is called Lamipol, has been described. This polarizing plate has a laminate structure of Al and SiO2, and according to this document, a very high extinction ratio is obtained. In addition, in J. Lightwave Tec. Vol. 15, No. 6, 1997, pp. 1042 to 1050, it has been disclosed that when Ge is used instead of Al which is responsible for the light absorption of Lamipol, a high extinction ratio can be realized at a wavelength of 1 μm or less. In addition, from FIG. 3 of the above document, it may be expected to obtain a high extinction ratio when Te (tellurium) is used. Although Lamipol is an absorption type polarizing plate having a high extinction ratio, as described above, since a laminate thickness of an absorption substance and a transmission substance determines the size of a light receiving surface, it is not preferably used for a projector polarizing plate which is requested to have a large size of several centimeters square.
In U.S. Pat. No. 6,122,103, a wire grid type polarizing plate has been disclosed. This polarizing plate is formed from fine metal wires disposed on a substrate at a pitch smaller than the wavelength of light in a service bandwidth, and predetermined polarization properties are obtained by reflecting a polarized light component parallel with the fine metal wires and by transmitting a polarized light component perpendicular thereto.
In addition, in U.S. Pat. No. 6,813,077, a method has been disclosed in which a wire grid type polarizing element having a three-layered structure is formed by forming dielectric layers and metal layers on a metal lattice so as to cancels light reflected from the metal lattice by an interference effect, and in which a wire grid, which is generally a reflection type, is used as an absorption type. It is believed that when an absorption type polarizing plate is used by utilizing the optical properties obtained from a multilayer structure, as described above, the thickness and the optical properties of the metal layer formed on the dielectric layer are important; however, in this patent document, these important properties are not taken into consideration. That is, in this patent document, the above important properties have not been described, and hence the details have not been known; however, in order to obtain the interference effect as described above, light is necessary to pass through the metal layer. When light passes, it means that in this step, part of the light is absorbed in the metal film located at an upper side. By the absorption, the transmittance in the transmission axis direction is decreased, and this decrease is not preferable as the properties of the polarization transmission axis; in particular, it is not preferable for a liquid crystal display device which is requested to have a high transmittance in a visible light region. That is, a polarizing plate having an absorption effect does not function when the optical anisotropic properties of an absorption layer are not essentially controlled and is difficult to be used as a practical polarizing plate.
In addition, in Japanese Unexamined Patent Application Publication No. 2006-323119, an inorganic polarizing plate in which semiconductor nanorods are dispersed in glass has been disclosed. It has also been disclosed that superior polarization properties are obtained in a visible light region; however, since this polarizing plate is formed by a method similar to that for Polarcor of Corning Inc., a drawing step is inevitably performed, and as a result, a large size plate is difficult to obtain.